1. Field of the Invention
This invention relates to offshore platforms. More particularly, it relates to a method for assembling the tendons used to moor a tension leg platform.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
A tension leg platform (TLP) is ideal for developing deepwater reserves. No other floating production facility design offers the optimal motion and stability characteristics of a tension leg platform. The TLP is vertically moored using tubular steel tendons and is supported by a buoyant hull. The tendon stiffness results in a system with virtually no heave, roll or pitch. This makes the TLP suitable for both dry tree and sub-sea completions.
A number of TLP solutions for deepwater field development have been designed, built and deployed around the world. The designs include both mono-column TLPs and multi-column TLPs.
The key benefits of a TLP are:                Minimum motion characteristics provide optimum support for risers and drilling/production equipment, and maximize personnel comfort and safety.        Vertical tendons provide small deepwater mooring profile and footprint thus allowing easy access around the platform for spread-moored drilling vessels and riser/umbilical installation vessels.        Scalable hull designs accommodate different payload requirements keeping design and engineering costs low for superior cost efficiency.        Modular, stiffened-plate hulls can be built in most shipyards or marine fabrication yards.        
All currently deployed TLPs have had their tendon systems installed by one of the following known methods:
Vertical stalking. Tendons are assembled offshore vertically at the surface by connecting joints of pipe vertically and lowering the assembly as additional joints are added to the string. The length of the joints is governed by the ability to handle and lift using a tall crane, and the availability of a tall assembly tower. Smaller facilities can be substituted when shorter joints are used, but assemblies of shorter joints require more time and more joints, and the cost of the couplings is increased.
One-piece wet tow. In this method, tendons are assembled by welding at a remote location (usually onshore) and subsequently towed in a horizontal orientation to the installation site. Tendons may be buoyant, neutrally buoyant or supported by floats to keep them at the surface. The top and bottom fittings are typically neither buoyant nor neutrally buoyant, and therefore require that floats be attached at each end of the tendon to provide support during the tow. Upon reaching the installation site, the tendon is upended by releasing selected floats while supporting the upper end from either the TLP or a support vessel.
Once the tendon is in the water and vertically oriented, the tendon can be either pre-installed by connecting its lower end to an existing anchor system and supporting its upper end with a temporary float, or co-installed by passing it over to the TLP to hang from the TLP vessel.